In recent years, there has been a significant amount of progress made in developing solid state laser systems for medical applications. Today, commercial systems are available which generate pulsed outputs at a variety of wavelengths. Many of the early solid state medical laser systems were based on neodymium doped gain media which generate a near infrared output wavelength of 1.06 microns. Using specific frequency selective optics, Nd:YAG medical lasers systems have also been developed which generate an output of 1.44 microns. (See, for example, U.S. Pat. No. 5,048,034).
It has been known for some time that tissue ablation can be enhanced through the use of infrared wavelengths that more closely match absorption peaks of water, the major constituent in biological tissue. To this end, the assignee herein (as well as others) have introduced medical laser systems which include a gain medium wherein holmium is the lasing species. These laser systems generate an output wavelength of 2.1 microns. The absorption coefficient in water for 2.1 micron radiation is more than two hundred times greater than the absorption coefficient of 1.06 micron radiation. This greater absorption coefficient means that more energy will be absorbed at the surface of the tissue (rather than penetrating into the tissue) resulting in cleaner ablation with less thermal damage.
One problem with the holmium laser systems is that they cannot easily be configured to generate the same output powers associated with Nd:YAG systems. To address this problem, multiple head laser systems have been developed, wherein each system includes two or more laser resonators, each having a holmium:YAG laser rod. The pulsed outputs of these laser resonators are interleaved and combined along a common output path to generate an output having a higher average power and higher repetition rate. Such system is described in U.S. Pat. No. 5,375,132, issued Dec. 20, 1994, assigned to the same assignee herein and incorporated by reference. (See also, U.S. Pat. No. 5,387,211) While the approach of using multiple resonators has been commercially successful, it is more complex and expensive than using a single resonator.
More recently, the industry has been exploring the use of erbium doped gain media for medical applications. Erbium doped YAG crystals will generate an output wavelength of 2.9 microns which is matched to a prominent absorption peak in water. Radiation at 2.9 microns has an absorption coefficient in water about two hundred times greater than 2.1 micron radiation. While this strong absorption in water would seem to make erbium laser systems the ideal candidate for tissue ablation in medical laser systems, certain problems arose.
One primary problem related to the lack of suitable optical fibers for delivering the 2.9 micron radiation. Common silica based fibers transmit 2.9 micron radiation with very low efficiency. Fluoride based fibers, which attenuate the 2.9 micron radiation to a much lesser extent than silica fibers, are relatively toxic to the human body. More recently, improvements in the fiber delivery technology have made the use of erbium laser light more viable.
Efforts have also been made to increase the pulse repetition rate of erbium laser systems having a single resonant cavity. Quite recently, a system was introduced which could generate a pulsed output at a maximum rate of 30 Hertz. Unfortunately, attempts to increase the repetition rate even higher result in a significant reduction in output power as well as an increase in the thermal loading of the laser rod leading to instabilities.